As is well known, polyacetal resins are conventionally employed in various end-use applications as engineering plastics due to their excellent physical properties, such as mechanical and electrical properties as well as their chemical and heat resistance properties. However, new end-use applications for polyacetal resins demand further property improvements.
For example, further improvement in the weather resistance of the polyacetal resin is required in some situations. In particular, conventional polyacetal resin used to form exterior parts of automobiles and components of electric and office appliances sometimes exhibit discoloration, surface smoothness deterioration (resulting in a reduction of the surface gloss), and/or crack formation on the surface of such parts when exposed to ambient conditions in their intended environment of use (e.g. solar (UV) light, rain, fluorescent lamps, and/or air). These deleterious effects not only impair the appearance of molded parts, but also adversely cause deterioration of the parts' mechanical properties.
It has been proposed to incorporate various stabilizers in polyacetal resins so as to improve their weather and/or UV-light resistance properties and thus overcome the above-noted effects. However, although some improved effects can be obtained by prior stabilization proposals, the surface conditions (e.g. cracks, reduced gloss, etc.) and the mechanical properties of molded polyacetal articles after being exposed to solar rays (ultraviolet rays), rain, and like outdoor elements are not always satisfactory. Thus further improvement in the weather/light resistance of polyacetal resin is needed.
A mere increase in the amount of the stabilizing agent in order to satisfy such a requirement only improves to a limited extent the weather/light resistance properties of polyacetal resin. In addition, increasing the amount of the stabilizing agent may impair the mechanical properties (such as tensile strength and bending strength of the polyacetal resin). Furthermore, stabilizing agents tend to form mold deposits during molding thereby reducing the dimensional accuracy and/or impairing the appearance of molded articles (not to mention the additional time needed to clean the mold) thereby reducing the efficiency of the molding operation. Another problem with conventional stabilizing agents is their tendency to "bleed" on the surfaces of molded articles thereby deleteriously affecting the articles' visual appearance.
According to the present invention, weather stabilized polyacetal molding compositions are provided having a major amount of a polyacetal resin, and a weather stabilizing effective amount of a stabilizing system which includes (i) at least one weather/light stabilizing agent, and (ii) an acrylate resin.
More specifically, according to the present invention, a weather-resistant polyacetal resin composition is comprised of (A) a polyacetal resin, (B) between about 0.01 to about 5 wt. %, based on the total weight of the composition, of a stabilizing agent and (C) between about 1 to 40 wt. %, based on the total weight of the composition, of an acrylate resin.